The present invention relates to key-operated door locks, and particularly to a protective device for application over such locks and including a guard plate for blocking access to the keyhole of the lock.
A number of protective devices for door locks have been proposed which include guard plates movable to a blocking position blocking access to the keyhole, or to an unblocking position permitting access to the keyhole. Examples of known devices of this type are described in U.S. Pat. Nos. 33,933, 3,732,711 and 4,365,491, and in German patents Nos. 223481 and 542922. The purpose of such devices is to provide protection against an attempted entry by someone using a key which had been fraudently copied, stolen or lost, or by someone using a master key or attempting to pick the lock. Thus, while such guard plates provide this additional protection, they have not yet come into widespread use because of the substantial inconvenience in operating them, and/or the substantial cost in producing them.
An object of the present invention is to provide a novel protective device of this type but one which is convenient and reliable to operate, and which can be manufactured in volume and at low cost.